If all horns are pitched in the same key, I would recommend trying SSA vocal arrangements, which are available for next to nothing at any well-stocked music store. Ignore the piano part and just assign the horns to each of the vocal lines. It's pop music, which will be a pleasant change from normal school stuff, and the arrangements (meant to feature teen girl vocalists in swing choir settings) are peppy and fun to play.
I used to use them for three clarinets (soprano, soprano and bass) as part of my teaching of clarinet students back in the 1980's. I'd have one student stay 10 minutes late, have the next show up 10 minutes early, and we all would have a good time.